The bending angle obtained by conventional sheet-bending presses may present considerable variations along the line of the bend. This arises mainly from the following causes: deflection of the press itself during its working action, particularly in the upper and lower tables, in the uprights, and in the tooling; tool wear; and tolerances in the manufacture of the press, its tools, and in the quality and thickness of the material to be bent. Furthermore, when two apparently identical sheets of metal are bent, they do not have perfectly identical characteristics, so that the same punch movement may produce substantially different bends.
In the systems currently known and used, various means are used partially to overcome these problems; some of them employ compensating jacks to compensate deflection during the working of the press and to obtain a constant bending angle throughout the length of the workpiece. French Pat. No. 2,119,528, for example, describes a press comprising compensating jacks in the the lower table; European patent application published as No. 0,025,469 describes a press fitted with compensating jacks in the upper table, and French patent application No. 2,200,064 describes a press with compensating jacks in both the upper and lower tables. Unfortunately, presses of this type require many test bends and adjustments to obtain a satisfactory result, and with most of them the operator has to pre-determine the amount of deflection that can be expected from the press. Furthermore, they compensate only those differences of bending angle caused by deflection of the machine, its uprights and upper and lower tables, and not other differences, such as those caused by local deflection or wear in the die or the punch. Nor do they solve the problem of repeating the bend to the same degree of accuracy in successive sheets of metal. For from batch to batch, and even within a single batch, the sheets of metal are not always rigorously identical, varying in thickness, resistance, or elasticity. Even within a single sheet, the characteristics may very from point to point. Thus for the same punch movement the bending angles obtained in two different sheets of metal, or even in different places in the same sheet of metal, may present measurable differences. To overcome these drawbacks during the bending cycle, the bending angle needs to be monitored while it is being obtained.
There are presses which enable the bending angle to be measured during the bending process. Such presses are described for example in French and German patent applications Nos. 2,362,722 and 2,044,199 respectively, and U.S. patent specification No. 3,440,847. However, the measuring devices described in these specifications are not very accurate since they measure the bending angle outside the die: the weight of the workpiece causes a considerable margin of error. A more precise measurement of the bending angle is obtained by means of the device described in British patent application No. 2,072,551, which enables the angle to be measured at the point of bending, in the die. However, it does not give accurate readings of asymmetric bending, for example, such as when a sheet is bent close to one of its edges, as one side of the bend is then much heavier than the other.
A more accurate device for measuring the bending angle is described in German utility model 82-34-901.0, which relates to a small press for bending metal sections. In this device, the bending angle is determined at the bending point by means of two measuring rods each of which have a flat surface against which the section is pressed during the bending process, the rods being so placed that they rotate as the section is bent. Measuring rods of similar shape can advantageously be used in a device for monitoring the bending angle in presses for bending sheet metal.